1. Field of the Invention
The proper performance of almost all electronic and many electro-mechanical devices is critically dependent on the characteristics of the AC power voltage supplied to operate them. Usually, these devices have tolerance built into them, so that a decrease in RMS voltage of even as much as 10% to 15% of nominal will not affect operation. However, operation degrades very rapidly once the designed-for tolerance is exceeded. Particularly in recent years, line power has become unreliable in this respect, with voltage being either deliberately reduced in what is commonly now known as a "brownout" or through outright failure of line power in a blackout. In the case of electronic computers and their associated peripheral gear, it is important that both of these conditions be detected relatively rapidly so that computation which might otherwise proceed erroneously can be halted with orderly system shutdown procedures until proper line or auxiliary power is available. It has been found that a computer can operate satisfactorily with an occasional power cycle or half cycle which dips below minimum operating value if the overall average is at or above this value, because of power stored in the power supply capacitors. To treat a single isolated instance of an AC wave cycle whose voltage dips below the desired minimum as a blackout would result in unnecessary interruption of processing. On the other hand, a series of low voltage cycles has a cumulative effect by eventual partial discharge of power supply filter capacitors to too low a level. It is therefore desirable to suspend operation if too many low voltage cycles occur within any arbitrary time period.
2. Description of the Prior Art
Detection of deviations in an AC power wave from the nominal peak value has been done in a variety of ways. Blackout detection is old in the art and is based on the assumption that AC power has failed once peak or RMS voltage drops below a preselected level. This level is usually selected such that time remains for the computer to go through an orderly shutdown procedure, thereby minimizing difficulties upon restart. U.S. Pat. No. 3,758,852 shows one embodiment of such apparatus. Other related U.S. Patents are U.S. Pat. Nos. 3,401,308 and 3,700,919.
Another aspect of AC wave analysis related to this invention involves the so-called disturbance analyzers as manufactured by, for example, Dranetz Engineering Laboratories, South Plainfield, New Jersey. These are "stand-alone" devices which note occurrence of disturbances in the AC line wave and provide an appropriate indication or warning to the operator.
U.S. Pat. No. 3,534,403 discloses error detector logic somewhat similar to that employed by this invention. However, the use is to provide a noise or error filter, rather than to determine an error condition.